The present invention relates generally to semiconductor devices, and more particularly to a semiconductor microprocessor or microcontroller with protection against low supply voltage.
Low supply voltage on a semiconductor microcontroller chip (device), that is, a supply voltage that falls below the minimum acceptable value or level necessary for proper operation of the chip or end system (i.e., necessary to avoid a malfunction), can be especially serious for sensitive applications, such as an automotive anti-lock braking system. For that reason, microcontroller chips have typically been provided with `brown-out protection`--protection against not a complete loss of power but a reduction in power supply level sufficient to cause malfunction of the circuit or system. Applications that require brown-out protection generally are systems with a controlled power supply and not battery driven.
Theoretically, brown-out protection should reset the microcontroller chip or system at any time that V.sub.DD falls below the minimum acceptable value. However, this minimum V.sub.DD value will depend upon the operating frequency of the device or other system constraints. For example, the critical value (V.sub.DDmin) may be 4.0 volts (V) at 20 megahertz (MHz) operation, and only 1.8 V at 4 MHz.
For that and other reasons, brown-out requirements may be qualified by market considerations, i.e., the specific application in which the device is to be used. Applications that require brown-out protection fall into many categories, such as appliances, industrial control, and automotive applications. Operational (plugged-in) appliances encounter brown-out situations during line voltage dip. Peripheral voltage sensitive elements will malfunction at that time even though the appliance may not be turned on for its primary function. Automotive electronics encounter brown-out situations when the ignition is turned on. For example, the automobile radio may be turned on by the ignition key just as the starter is engaged, causing the power supply to drop. The affected device must reset or it may be left in an unstable state.
Brown-out is not applicable in low voltage battery applications where power consumption, operating frequency, and noise are at a relative minimum.
One may then define brown-out for the situation in which V.sub.DD falls to or below a predetermined voltage threshold level. Because microcontrollers operate at high speeds, it is important to also distinguish a real brown-out from normal I/O switching noise which may last as long as 200 nanoseconds (ns). In the circumstances noted above, for a 5 V supply, for example, if V.sub.DD falls below a level of 4.0 V for more than 200 ns, then brown-out detect should trigger to reset the chip.
Problems arise, however, in situations of relatively shallow dips of the supply voltage below the specified voltage threshold level, whether as an isolated excursion or a sequence of slight excursions of V.sub.DD below and then back above the threshold. In such instances, whether attributable to noise, I/O switching, or other line perturbations, the brown-out detection circuit typically initiates an immediate reset of the microcontroller chip on the first occasion and, if there is no further recurrence within a set period, reestablishes operation of the chip promptly after the set period expires. This, of course, results in disruption of the operation of the microcontroller when there is no need for a reset. Also, if V.sub.DD drops to the trip voltage and remains there for a lengthy period, the circuit may oscillate with attendant unstable operation.
It is a principal object of the present invention to provide a brown-out detection circuit which is capable of analyzing the nature of supply voltage dips, and of applying predetermined criteria and hysteresis to the determination of whether or not a reset should be initiated each time the supply voltage drops below a threshold level.